Catalyst for nh4no3 combustion



United States Patent C) 3,028,273 CATALYST FOR NH N COMBUSTION Wayne A. Proell, Chicago, Ill., assignor to Standard Oil Company, Chicago, 111., a corporation of Indiana No Drawing. Filed May 17, 1952, Ser. No. 288,549

Claims. (Cl. 149-46) This invention relates to new and improved explosive compositions and also to compositions for the generation of gas at high pressure, Particularly, the invention relates to explosive compositions wherein ammonium nitrate is the principal or sole gas-producing agent. Still more particularly, the invention relates to an explosive comprising ammonium nitrate, an oxidizable material and a particular combustion catalyst.

Ammonium nitrate is Widely used as a component of high explosives. Although ammonium nitrate is classified as a high-explosive, it is extremely insensitive and cannot readily be detonated by the local application of heat or by a blasting cap; and when ignited, ammonium nitrate does not sustain propagation consistently. Normally ammonium nitrate is mixed with an oxidizable material, such as, sulfur, carbon, cellulosic materials, hydrocarbons, etc., in order to utilize the excess oxygen available in the ammonium nitrate. However, these mixtures of ammonium nitrate and oxidizable materials are also either very insensitive or slow burning.

One of the well known methods of overcoming this lack of sensitivity is to use a sensitive high explosive to prime the detonation of the ammonium nitrate explosive. Examples of suitable primers are tetryl, TNT, nitrostarch, nitrocellulose, nitroglycerine, etc. An explosive that is detonable by the action of a blasting cap can be obtained by mixing the ammonium nitrate with the sensitive materials, such as, nitrostarch and nitrocellulose. The extreme sensitivity of these explosives makes them undesirable for ordinary blasting use.

Another method of obtaining a readily ignitable ammonium nitrate explosive is to admix therewith an organic sensitizer, such as, nitrogen compounds and certain carbohydrates. In general, satisfactory explosives are obtained only when the sensitizer is very intimately dispersed throughout the mass. Generally this dispersion is obtainable only by the use of complicated and expensive procedures.

The most commonly used method for improving the sensitivity of ammonium nitrate explosives is to add a combustion catalyst. The commercially used combustion catalysts are all based on the element chromium. The more common chromium combustion catalysts are ammonium or alkali metal chromates or polychrornates; chromic oxide, chromic nitrate and copper chromite. The preferred material is ammonium dichromate. While the chromium compounds are the best known combustion catalysts, they have the disadvantages of being expensive and of, frequently, being in very short supply. The relative toxicity of the chromates makes them hazardous unless handled with considerable care. A particular disadvantage to the use of chromium combustion catalysts is their strong oxidizing properties, such that they tend to react with the binder upon long storage to give chromium salts which are relatively inefiective catalytically.

It has been discovered that certain iron compounds are efiective catalysts for the combustion of ammonium nitrate and ammonium nitrate-oxidizable material mixtures. These catalysts are the subject matter of US. patent applications filed by Wayne A. Proell and William G. Stanley, Serial Number 273,564, filed February 26, 1952, now US. Patent No. 2,987,389, and Serial Number 288,065, filed May 15, 1952, now US. Patent No. 2,955,033. All of the combustion catalysts disclosed in these applications contain the iron cyanide radical, either ferrocyanide or ferricyanide. In addition to the iron cyanide radical, these catalysts contain a second iron ion which may be either ferric or ferro(us). In addition to the iron-iron cyanide complex, the catalyst may contain alkali metal and/or ammonium ions. It has been found that the generic classes of iron-iron cyanide compounds known as soluble Prussian blues and insoluble Prussian blues are effective catalysts for the purposes of this invention. It is known that the better soluble Prussian blues contain alkali metal(s) such as potassium and sodium and/ or the ammonium radical. Some of the compounds which have been found to be effective are: ferro ferrocyanide, ferric ferrocyanide, ferro ferricyanide, ferric ferricyanide, potassium ferric ferrocyanide, sodium ferric ferrocyanide, ammonium ferric ferrocyanide, potassium soluble Pressian blue, sodium soluble Pressian blue and ammonium-sodium soluble Pressian blue.

The above applications disclose that insoluble Prussian blue is the more effective catalyst at elevated pressures such as exist in the combustion chamber of a rocket motor, e.g., 500 to 2000 psi. However, the insoluble Prussian blue containing ammonium nitrate-oxidizable material mixture is very difiicult to ignite at low pressure, e.g., about 15 psi when the catalyst content is below about 6 weight percent. The soluble Prussian blue containing mixture will ignite readily at low pressure when using as little as 3 to 4% of the catalyst, but the combustion is not self-sustaining in an inert atmosphere.

An object of this invention is the preparation of a new and improved inorganic nitrate explosive, in particular, an ammonium nitrate explosive. Another object is the preparation of an explosive comprising ammonium nitrate and a combustion catalyst. Still another object is to provide a combustion catalyst for ammonium nitrate explosives which is cheap. A particular object of this invention is an explosive mixture comprising ammonium nitrate, an oxidizable material and a particular burning catalyst, which mixture can be formulated by a hot melt process. A further object of this invention is a gas generating composition for the generation of gases at pressures on the order of 1000 p.s.i., which composition comprises ammonium nitrate, an oxidizable material and an ammoniated-insoluble Prussian blue combustion catalyst.

The above objects and other objects which will be apparent in the detailed description of the invention are achieved as follows: The gas generating composition of this invention comprises a mixture of ammonium nitrate, or a mixture of ammonium nitrate and an oxidizable material, and an effective amount of a combustion catalyst consisting essentially of ammoniated-insoluble Prussian blue. In addition to the ammonium nitrate, moderate amounts of other inorganic nitrates, such as, potassium nitrate, sodium nitrate and magnesium nitrate may be present. It is preferred to have present in the high pressure gas generating composition of this invention an oxidizable material, which material utilizes a part or all of the excess oxygen available from the decomposition of the ammonium nitrate and the combustion catalyst. (The catalyst uses some of the excess oxygen from the decomposition of the ammonium nitrate for the oxidation of the metal and carbon content thereof.)

The combustion catalyst of this invention is obtained by exposing ferric ferrocyanide or an insoluble Prussian blue to the action of free-ammonia. The ammonia is rapidly taken up for a time; the rate of addition decreases and finally stops. The point of completed ammonia gas addition is indicated fairly well by the fact that the temperature of the reaction zone reaches a constant level. The ammoniated material has a strong odor of ammonia if cooled to room temperature quickly after the interaction of ammonia and the material has come to an equilibrium, as indicated by a constant reaction zone temperature. The amount of ammonia taken up will vary somewhat with the particular material charged. However, it will be between about 5 and 8 weight percent, based on iron-iron cyanide starting material. The odorous catalyst has the ordinary pressure ignition properties of soluble Prussian blue catalyst and the highpressure burning rate properties of soluble Prussian blue catalyst.

Not all the ammonia which is taken up is firmly bound by the iron-iron cyanide material. It has been found that prolonged heating of the odorous catalyst at a temperature of about 65 C. will result in the elimination of substantially all the ammonia odor. The amount of ammonia retained, apparently chemically bound, by the ironiron cyanide material varies with the type of material used in the ammoniation process; the amount of ammonia present in the odor-free catalyst is between about 3 and 5 weight percent, based on iron-iron cyanide material. The odor-free catalyst and the odorous catalyst are substantially equivalent in catalytic effect.

The term iron-iron cyanide material is intended to include ferric ferrocyanide and all the various grades of insoluble Prussian blue. The term ammoniated-insolubl: Prussian blue is intended to include the product of the ammonia treatment of iron-iron cyanide material whether or not the product has been free of adsorbed ammonia to produce a substantially odor-free ammonia containing catalyst.

By the use of this ammoniation procedure for insoluble Prussian blue, it is possible to overcome the two disadvantages of ordinary insoluble Prussian blue as a combustion catalyst for ammonium nitrate. The ammoniated-insoluble Prussian blue readily ignites at low pressures. Insoluble Prussian blue as obtained on the commercial market shows a considerable variation in catalytic activity from source to source and even from batch to batch from the same source; the ammoniated-insoluble Prussian blue is of about the same catalytic activity regardless of the source of the insoluble Prussian blue used as the starting material in the ammoniation procedure.

When operating with ammonium nitrate in the absence of any appreciable amount of oxidizable material, large amounts of catalyst are needed to permit smooth burning of the composition. For ignition at ordinary temperatures, the amount of catalyst needed may be as much as 10 weight percent of the total explosive mixture. However, the ease of ignitability of the mixture increases with increase of pressure in the gas generating chamber and at elevated pressures, ignition can take place with the use of smaller amounts of combustion catalyst. The term ordinary pressure is intended to mean pressure such as exists normally in the atmosphere or in mining operations.

When the gas generating composition consists essentially of ammonium nitrate, an oxidizable material and the combustion catalyst of this invention, the combustion catalyst may be present in an amount between about 0.5 and 25 weight percent, based on the total composition. In order to improve the ignitability of the composition and obtain smoother burning thereof, it is preferred to use at least about 2% of catalyst. The burning rate is somewhat affected by the amount of catalyst present in the composition; however, the maximum effective concentration of the catalyst is about The amount of catalyst needed to obtain good ignitability and smooth operation is dependent somewhat on the amount and type of oxidizable material present; in general between about 2 and 4 Weight percent will give excellent results when maximum burning rate is not a main consideration. When maximum burning rate is a main consideration, the amount of catalyst used should be between about 6 and 15 weight percent.

The decomposition of ammonium nitrate produces freeoxygen in addition to the other decomposition products.

Additional energy can be obtained by having present in the combustion zone an oxidizable material which combines with the free-oxygen. It is to be understood that the combustion catalysts utilized in the explosive grain of this invention contain oxidizable materials which will consume some of the free-oxygen. The oxidizable material may be any material which contains a deficiency of combined oxygen. Metals such as aluminum and magnesium may be used. The non-metallic elements, sulfur and carbon, may be used. Nitrogen-containing organic compounds that do not unduly sensitize the explosive mixture are particularly good; examples of these are urea, nitroguanidine, mononitrate naphthalene, dinitrodiphenyl oxide, etc. Hydrocarbon materials are an excellent oxidizable material and are preferred where low cost is a matter of concern. Examples of hydrocarbon oxidizable materials are waxes, tars, asphalts, bitumen, coal tar, shale oil residue, viscous higher boiling hydrocarbon oils, etc.

Oxygenated materials are very useful as oxidizable materials for certain applications of the explosive grain. For use in rockets, a material that is dimensionally stable is desirable. It has been found that the various cellulose acetates which have been plasticiled to improve workability are a particularly suitable oxidizable material. It is desirable to use an oxygenated plasticizer and a polyester of a glycol and a dicarboxylic acid is a particularly suitable plasticizer material. The term oxygenated oxidizable material is intended to include all organic materials which contain oxygen as well as carbon and hydrogen-other elements such as nitrogen and sulfur may also be present.

The amount of oxidizable material that is added to the ammonium nitrate-catalyst mixture is dependent upon the amount of catalyst present and the particular type of oxidizable material itself. Normally it is desirable to have present a sufficient amount of oxidizable material so that the explosive mixture is about stoichiometrically balanced with respect to oxygen content. This preferred composition may be exceeded, but in general no more oxidizable material should be present than can react with the available oxygen to yield a soot-free gas. The presence of some oxidizable material is desirable so that the explosive mixture of this invention should contain between about 5 and 25% of oxidizable material, preferably between about 15 and 25% of oxidizable material.

The term ammonium nitrate as used in this specification and in the claims is intended to mean either ordinary commercial grade ammonium nitrate, such as, conventionally grained ammonium nitrate containing a small amount of impurities and which is then generally coated with a small amount of moisture-resisting material such as petrolatum or parafiin, or military grade ammonium nitrate, or a mixture of other inorganic nitrates and ammonium nitrate wherein the ammonium nitrate is the preponderate nitrate.

The explosive mixture can be made by milling the ingredients or by dry mixing; this operation is preferably followed by forming regular shaped compacted grains by pressing the powdered mix in molds. It is preferred to prepare the shaped grains by adding the powdered ammonium nitrate to fused organic or oxidizable material at l25 C., mixing to form a paste, and pressing the paste into suitable molds. The cooled grains are strong and durable.

The effectiveness of ammoniated-insoluble Prussian blue as a combustion catalyst is illustrated below. Fortythree grams of an insoluble Prussian blue (essentially pure ferric ferrocyanide) was placed into a flask. The flask and contents were at room temperature, about 20 C. Free-ammonia gas was slowly passed through the finely powdered mass of insoluble Prussian blue. The temperature of the reaction zone as measured by a thermometer inserted into the powder immediately increased and continued to increase fairly rapidly to about a temperature of about 60 C. The temperature increase slowed down at this point and slowly rose to about 65 C. and became substantially constant at that point. The ammonia gas stream was turned ofi when the reaction zone temperature leveled out. The flask and its contents were cooled to room temperature. The physical appearance of the insoluble Prussian blue had not been changed by this treatment. The powder was weighed and found to have increased in weight by 2.8 g. or 6.5%, based on insoluble Prussian blue charged. The ammoniated insoluble Prussian blue had a very strong odor of free-ammonia at room temperature even after many hours standing in an open beaker.

The odorous material was placed in a beaker and allowed to stand in a water bath at a temperature of about 65 C. until the material had reached constant weight. Weighing showed that the ammoniated material had lost a total of 1.0 g. Thus, 1.8 g., 4.2 weight percent, of freeammonia was firmly bound by the insoluble Prussian blue. The lower ammonia content material had substantially no ammonia odor at room temperature.

Test grains of the desired composition were made as follows: When necessary, the ammonium nitrate was ground in a mortar to break up small lumps. The desired amount of ammonium nitrate, oxidizable material and combustion catalyst were weighed into a beaker and the contents thereof were thoroughly mixed. The mixture was extruded by means of a laboratory-size extruder to form a grain about /2 inch in diameter and 6 inches long. In order to insure uniformity, duplicate grains were made and tested in each example. The burning characteristics of each explosive mixture were determined after inhibiting the cylindrical surface of the grain with a thin layer of either asphalt or a cellulose base thermoplastic such as cellulose acetate. Burning rates were determined at atmospheric pressure by igniting the end of the grain and placing a beaker over the ignited grain, thus simulating burning in an inert atmosphere. For tests at elevated pressures, the gain was placed in a Crawford bomb and the bomb brought to the desired operating pressure by means of cylinder nitrogen prior to igniting the grain. Each burning rate represents an average of at least two trials.

Test grains were made by incorporating 6 weight percent of catalyst in a standard ammonium nitrate-oxidizable material mixture. The total explosive mixture consisted of ammonium nitrate, 70 weight percent; cellulose acetate, 6%; polyester of ethylene glycol and diglycolic acid, 9%; dinitrodiphenyl oxide, 9%; catalyst, 6%.

Test 1 In this test grains of the above composition were made up using odorous ammoniated-insoluble Prussian blue as prepared above, and the insoluble Prussian blue charged to the ammoniation process. The composition using the insoluble Prussian blue would not burn at atmospheric pressure. The composition using the ammoniated insoluble Prussian blue ignited readily and burned at a rate of 0.01 in./sec. Both grains ignited readily at 1000 p.s.i. in the Crawford bomb and gave a burning rate of 0.13 in./sec.

Test 2 In this test the catalyst was odor-free ammoniated insoluble Prussian blue, as prepared above. The grain ignited readily and gave a burning rate of 0.01 in./sec. This grain gave a burning rate of 0.13 in./sec. in the Crawford bomb at 1000 psi.

Test 3 In order to note the eflect of high temperature on the preparation of the ammoniated insoluble Prussian blue, an amount of material was prepared as follows: Insoluble Prussian blue as used above was placed in a flask and a stream of ammonia gas passed through the powder while the reaction zone was maintained at a temperature of about 300 C. A considerable amount of water was evolved from the reaction zone. The contents of the flask were cooled to room temperature. The reaction product was a fine black powder which was pyrophoric; however, upon aging in a closed container overnight, the pyrophoric property disappeared. The reaction product of this test was used in the above described test composition and gave a burning rate at atmospheric pressure of 0.1 in./sec. It is indicated that substantially no improvement in catalytic effectiveness is obtainable by carrying out the ammoniation procedure at a temperature above the plateau point of about C.

I claim:

1. An explosive composition which comprises a predominant amount of ammonium nitrate, an efiective amount of a combustion catalyst, which catalyst is prepared by treating insoluble Prussian blue with ammonia at a temperature between about 20 and about 300 C. until essentially no further absorption of ammonia by the Prussian blue takes place.

2. The composition of claim 1 wherein the ammonia absorbed is between about 3 and 8 weight percent based on Prussian blue charged.

3. An explosive composition comprising between about 5 and 25 weight percent of an oxidizable material, between about 0.5 and 25 weight percent of a combustion catalyst and the remainder essentially ammonium nitrate wherein said combustion catalyst is prepared by contacting insoluble Prussian blue with ammonia at a temperature between about 20 and about 300 C. for a time sufiicient to permit the insoluble Prussian blue to absorb between about 3 and about 8 weight percent of ammonia.

4. The composition of claim 3 wherein said catalyst is prepared by contacting insoluble Prussian blue with ammonia at a temperature of about 65 C. for a time sufficient to permit the insoluble Prussian blue to absorb about 6 weight percent of ammonia.

References Cited in the file of this patent UNITED STATES PATENTS 43,021 Halvorson June 7, 1864 1,021,882 OBrien Apr. 2, 1912 1,071,949 OBrien Sept. 2, 1913 1,890,112 Fisher Dec. 6, 1932 2,159,234 Taylor May 23, 1939 FOREIGN PATENTS 14,196 Great Britain 1897 

1. AN EXPLOSIVE COMPOSITION WHICH COMPRISES A PREDOMINANT AMOUNT OF AMMONIUM NITRATE, AN EFFECTIVE AMOUNT OF A COMBUSTION CATALYST, WHICH CATALYST IS PREPARED BY TREATING INSOLUBLE PRUSSIAN BLUE WITH AMMONIA AT A TEMPERATURE BETWEEN ABOUT 20* AND ABOUT 300*C. UNTIL ESSENTIALLY NO FURTHER ABSORPTION OF AMMONIA BY THE PRUSSIAN BLUE TAKES PLACE. 